Antiskid control—combined paired/individual wheel control logic

ABSTRACT

An anti-skid brake control system for a multi-wheeled vehicle includes both a paired function and an individual function. The paired function controls the wheels of the vehicle in unison. The individual function controls the wheels of the vehicle individually. A paired/individual logic circuit alternatively activates and deactivates the paired function and the individual function. A method for controlling the skid of a vehicle utilizing a paired function and a individual function is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Divisional Application of Continuation application Ser. No.11/777,047, filed Jul. 12, 2007, now U.S. Pat. No. 7,775,607, which is aContinuation of application Ser. No. 10/986,559, filed on Nov. 11, 2004,now U.S. Pat. No. 7,258,404, the entire contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to anti-skid braking systems forwheeled vehicles. More particularly, this invention relates to animproved system and method for controlling wheel slip in a multiplewheel braking system. This improved system is particularly useful onvehicles with a wide wheel base such as general aviation aircraft. Thisimproved system optimizes the braking performance of such pairedmulti-wheeled vehicles while minimizing directional deviation.

GENERAL BACKGROUND AND STATE-OF-THE-ART

Anti-skid and automatic braking systems commonly have been provided onmulti-wheeled vehicles such as general aviation and commercial aircraftto aid the deceleration of the vehicle. Modern anti-skid systemstypically optimize braking efficiency by adapting to runway conditionsand other factors affecting braking to maximize deceleration,corresponding to the level of brake pressure selected by the pilot. Inconventional antiskid systems, brakes are typically applied mechanicallyvia a metering valve by the pilot, and as soon as the wheel brakepressure approaches the skid level, such as when an initial skid isdetected, a brake pressure value is used to initialize the antiskidcontrol system.

In aircraft applications, rapid pedal application by an aircraft pilotduring landing can often create deep initial skids before an effectiveanti-skidding brake pressure or brake torque is determined and skiddingis effectively controlled by conventional antiskid and brake controlsystems. Reducing initial skids and maximizing braking efficiency wouldresult in shorter aircraft stopping distances, which allow the aircraftto land on shorter runways, and can result in reduced tire wear.

Conventional skid control systems typically include slip indicatorshaving a wheel speed transducer and a brake pressure sensor for eachwheel brake of the vehicle. The wheel speed transducers measure wheelspeed and generate wheel speed signals that are a function of therotational speed of the wheel. The wheel speed signals are typicallyconverted to a signal representing the velocity of the wheels, andcompared with a reference velocity of the aircraft. This comparison maygenerate a wheel velocity error signal indicative of the differencebetween the wheel velocity signals from each braked wheel and thereference velocity signal. The output of the velocity comparator isreferred to as “velocity error.” The velocity error signals typicallyare adjusted by a pressure bias modulator integrator, a proportionalcontrol unit, and a compensation network. The output of such logiccircuits are summed to provide an anti-skid control signal which isreceived by a command processor. The pressure bias modulator integratordictates the maximum allowable control pressure level during braking.That is, when no skid (or slip) is detected, this integrator allows fullsystem pressure to the brakes and allows less pressure as the skid isdetected.

Conventional and recently invented anti-skid control systems arewell-known to those of ordinary skill in the art. Some recently inventedanti-skid control systems are described in U.S. Pat. Nos. 4,562,542;6,655,755; and 6,659,400. Those of ordinary skill in the art ofanti-skid brake control are familiar with the advantages anddisadvantages of the different control systems described in these andother patents.

Conventional anti-skid systems have processed the pressure bias signalin at least two different ways: either as paired or individual controlsignals. In paired skid control systems the skid control signal from asingle wheel is sent to both wheel's pressure control valve. Inindividual skid control systems, each wheel generates an individual skidcontrol signal which is sent to its own pressure control valve.

Paired skid control systems typically take either the signal from thefirst wheel to indicate a slip or from the wheel indicating the greatestslip and uses that signal to modulate the brake pressure to both or allwheels. Such systems prevent directional deviation from being generatedby the anti-skid system. In vehicles with wide wheel bases, such asgenerate aviation aircraft, small variations in the brake pressurebetween the spaced apart wheels may induce directional deviation. Onedrawback of these systems is reduced braking efficiency. Since the brakepressure to all wheels is being reduced in response to the wheelexperiencing the lowest friction coefficient, available brake pressureto the other wheels is being sacrificed.

Individual skid control systems take the signals from each individualwheel and responds with a signal to modulate brake pressure sent to eachindividual wheel's pressure control valve. That is, the control systemmodulates the brake pressure to each individual wheel according to theslip conditions experienced by each wheel. These systems maximize thebraking efficiency of the entire vehicle by allowing the maximum allowedbraking pressured for the conditions experienced by each wheel. Onedrawback of these systems is directional deviation caused by theanti-braking system. As greater brake pressure is applied to a wheelexperiencing less slip, torque is generated by the different forcesapplied to different wheels. Torque on the vehicle is experienced asdirectional deviation.

A need therefore exists for an anti-skid braking system for amulti-wheeled vehicle which optimizes the braking efficiency of thevehicle while minimizing directional deviation. The present system meetsthese and other needs.

INVENTION SUMMARY

Briefly and in general terms, the present invention provides for animproved system and method for anti-skid braking control in amulti-wheeled vehicle. This invention can be applied to any vehicle thatuses a powered brake control system to control vehicle deceleration. Alogic circuit is provided with the anti-skid braking control system toselectively initiate either a paired function or individual function onthe signals sent to the brake pressure valves. As described herein, a“function” is a general term for the process performed by a logic orother electronic circuit. This logic circuit evaluates the slipcondition on each wheel to evaluate the proper form of control for thevehicle. When the logic circuit selects the paired function, theanti-skid system provides the same signal to each of the brake pressurevalves. This signal typically is in response to the conditionsexperienced by the first wheel to indicate a slip condition (known asthe “lead wheel.”) When the individual function is selected by the logiccircuit, the anti-skid system provides different signals to each wheel'sbrake pressure valve. These signals are in response to the conditionsexperienced by each individual wheel.

The paired/individual logic circuit works in conjunction with ananti-skid control system on a multi-wheeled vehicle. Each wheel includesa set of brakes controlled by its own brake control valve. Typically,these brakes are hydraulically actuated and the brake control valveregulates the hydraulic pressure delivered to the brakes. Electrical andpneumatic brake systems are also contemplated. Within the anti-skidbrake control system, each wheel also has a slip indicator. Typically,these slip indicators include a wheel speed transducer which generatesan electronic signal indicating wheel speed (V_(w)). This signal is thencompared with a reference velocity signal (V_(ref)) for the vehicle. Anelectronic comparator indicates whether the wheel is experiencing slipor skid. When the wheel velocity and the reference velocity aredifferent a velocity error signal is initiated. The velocity comparatoris also configured to generate a signal indicative of the degree of slipor the slip velocity (Vs). A deep slip has a greater Vs than a minorslip.

Braking induced slip (or skid) generally occurs when the braking forceon an individual wheel exceeds the friction force between the wheel andthe road (or runway) surface. The amount of friction force is largelydetermined by the wheel/surface coefficient of friction (μ). Thecoefficient of friction is a measure of the amount of friction availablebetween the material of the tire and the material of the surface.Coefficient of friction also varies with a number of factors includingtire wear, surface conditions, temperature etc. Thus, even on a singlevehicle the available friction force between the tire and the surfacecan vary between individual wheels. This results in differing slipconditions for each wheel upon braking. Therefore, the use of individualslip indicators is needed on individual wheels to accurately monitorslip conditions.

In the present invention, the anti-skid brake control system monitorsthe signals from each wheel's slip indicator. When a slip condition isfirst indicated, the system designates the first wheel to indicate theslip as the “lead wheel.” The next wheel, and any other later wheel, isdesignated the “follower wheel.” After the first slip condition has beenindicated the paired/individual logic circuit provides the option ofcontrolling both (or all) tires in unison or individually.

In a currently preferred embodiment, the paired/individual logic circuitinitiates the anti-skid control signals as a paired function. Thisenables the anti-skid brake control system to calculate the neededbraking force dependent on the slip velocity of the lead wheel. Thesystem provides this information to the brake control valve of eachwheel. These signals are typically very dynamic, adjusting the amount offorce applied to the brakes many times a second. While the pairedfunction is active, the anti-skid braking system continues to monitorthe slip velocity of the all wheels but provides braking signalsaccording to lead wheel activity.

With the paired function active, the brake command to each wheel isadjusted in the same way at the same time. This is particularlyadvantageous on vehicles with a relatively wide wheel base such asgeneral aviation aircraft. On such vehicles, small variations in brakingforce between the spaced apart wheels can result in directionaldeviation. Uniform brake pressure on all wheels minimizes thiscondition.

While the paired function is active, the system also continues tomonitor the signals generated by each wheel. Under certain conditions,the paired/individual logic circuit will override the paired function.An excessive slip velocity indication by a follower wheel is one suchcondition. A slip velocity may be excessive if it is greater than theslip velocity indicated by the lead wheel or if it exceeds a referenceslip velocity. The reference slip velocity may be set to preventunacceptably inefficient braking performance.

With the individual function active, the brake command to each wheel isadjusted individually in response to the slip velocity indicated by eachwheel. This permits both maximum brake efficiency on each wheel andresultant maximum brake efficiency for the vehicle. That is, brakecommand on each wheel may be reduced (or increased) just enough toalleviate the slip condition experienced by each wheel. These signalsare also highly dynamic, adjusting the brake command to each wheel manytimes per second.

In a presently preferred embodiment, once the individual function is setit continues to control the anti-skid brake control system until theslip conditions desist or the vehicle stops altogether. Alternatively,the paired/individual function may reinitiate the paired function inresponse to resultant conditions. For example, once each wheel's slipcondition falls below the reference slip velocity, or possibly if adirectional deviation due to variation in brake command is experienced.

The paired/individual logic circuit may also be configured to initiatethe individual function at the first indication of a slip condition. Forexample, if the initial indication of a slip condition exceeds thereference slip velocity, the paired/individual logic circuit mayinitiate the individual function without prior resort to the pairedfunction. This may occur when a deep slip results in greaterdifficulties in vehicle control than would be experienced by thedirectional deviation caused by individual wheel control.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description, and the accompanyingdrawings which illustrate, by way of example, the features of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the anti-skid brake system of the presentinvention.

FIG. 2 is a schematic of the logic employed by the paired/individuallogic circuit of the present invention.

FIG. 3 is a schematic of the velocities and forces experienced by ageneral aviation aircraft while decelerating.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Many vehicles have a need for anti-skid brake controls for rapiddeceleration. Aircraft need such systems particularly for landing.Anti-skid brake systems for general aviation aircraft have typicallybeen of two types. The first type, a paired wheel control system, sendsthe same braking control signals to both braking wheels. The secondtype, an individual wheel control system, sends different signals toeach wheel. Both of these systems have characteristics which may lead toundesirable braking performance. A novel combination of these systemsunder appropriate circumstances results in minimizing these undesirablecharacteristics.

With reference to FIG. 1, in an anti-skid brake system 20, of amulti-wheeled vehicle, the system includes a pair of wheels: wheel 1 andwheel 2. The wheels on a general aviation aircraft include mechanicalbrakes which are activated hydraulically. In such a system the hydraulicpressure to each brake is controlled by a brake control valve 22, 24.

Each wheel 1, 2 of the anti-skid brake control 20 is equipped with anelectronic slip indicator 26, 28. Each slip indicator is generallycomprised of a velocity transducer 27, 29 which generates an electronicsignal indicative of the wheel velocity (V_(w)). Wheel velocity may bemeasured in radians per second to indicate rotational speed. The slipindicator has an electronic comparator which compares the wheel velocityto a reference velocity of the vehicle (V_(ref)). If the comparatorindicates that the wheel velocity is lower than the vehicle referencevelocity then a slip condition is indicated. The electronic comparatoralso measures the amount of slip and calculates a slip velocity (V_(s)).Slip velocity is calculated as a function of the difference between thewheel velocity and the vehicle reference velocity. V_(s)::V_(ref)−V_(w).Therefore, deeper slips have a higher slip velocity than minor slips.

Slip indicators 26, 28 may also include brake pressure sensors (notshown) which generate signals that are a function of the brakingpressure applied to the wheel brake. These sensors measure the amount ofpressure resulting from the condition of the brake control valves 22,24. As the brake control valves open, the pressure between the rotatingportion of the brakes (rotors) and stationary portion of the brakes(stators) increases with the increased hydraulic pressure. The brakepressure signal may be compared with a threshold brake pressure. Thiscomparison along with the signals from the velocity transducer may beused to ensure maximum efficiency of the braking process.

In accordance with the present invention, the slip indication and theslip velocity signal from each wheel are sent from the slip indicators26, 28 to the anti-skid brake control system 30. The anti-skid brakecontrol system includes a paired/individual logic control circuit 32.Upon receipt of the initial indication of slip, the paired/individuallogic circuit designates the wheel sending the initial signal as the“lead wheel.” The paired/individual logic circuit also determineswhether to control the skid using a paired control function 34 or anindividual control function 36. Thus, the functions of thepaired/individual logic circuit are to a) assign a lead wheel and b)switch the function of the anti-skid brake control system between apaired control system and an individual wheel control system. Thesefunctions are used by the anti-skid brake control system to provideanti-skid control to each brake control valve 22, 24.

The signals from each wheel's slip indicator 26, 28 is also sent toindividual wheel control circuits 38, 40. Each of these control circuitscalculates the optimum brake force to apply to control the slipexperienced by the individual wheel. Typically, these control circuitsgenerate signals to reduce the brake force applied to a wheelexperiencing slip. The optimum brake force is the force that is just lowenough to prevent the slip experienced by the wheel, and high enough tomaximize the deceleration of the vehicle. The control circuits may alsoreceive electronic signals from the cockpit brake pedals. Increasedpedal pressure on the brake pedals indicate a greater requirement fordeceleration. The control circuits continually calculate the need fordeceleration and anti-skid control and send out signals to modulate thebrake pressure many times a second. Those of skill in the art ofanti-skid braking are familiar with a variety of control circuits whichperform these functions. Any such control circuit is contemplated foruse in the present invention.

The signals from the wheel control circuits 38 and 40 pass through theanti-skid brake control system 30. If the individual function 36 isactive, then the signals from each wheel control circuit is sentdirectly to the corresponding wheel's brake control valve. In thismanner, each wheel control circuit continuously regulates the brakeforce applied by the corresponding wheel in direct response to the slipsignals generated by that wheel.

If the paired function 34 is active, the anti-skid brake control system30 filters out the signals sent by the wheel control circuits of all butthe lead wheel. The control signal of the lead wheel's control circuitis sent to each wheel's brake control valve. In this manner, the signalsfrom the lead wheel designated by the paired/individual logic circuit 32continuously regulates the brake pressure in all wheels.

The signals sent to the brake control valves 22, 24 either through thepaired function 34 or the individual function 36 of the anti-skid brakecontrol system 30 continuously regulates the braking force on eachwheel. Typically, brakes are hydraulically controlled and therefore thebrake control valves are hydraulic valves. These valves control thehydraulic pressure sent to the brake actuators. Increased hydraulicpressure results in increased braking force. Pneumatic systems mightwork in the same manner using pneumatic valves as the brake control.Potentially an electrically actuated braking system would have anelectrical control as a valve. Conceptually each of these systems wouldwork in the same manner.

With reference to FIG. 2 the paired/individual logic circuit 32evaluates various conditions to determine when to set the pairedfunction 34 or the individual function 36 as active. Signals from theslip indicators 26, 28 of each wheel including the slip velocity signalare sent to the paired/individual logic circuit. The first signal thatindicates a slip begins the process. The paired/individual logic circuitassigns the first wheel to indicate a slip condition as the “leadwheel.” The other wheel is considered the “follower wheel.” On vehicleswith more than two braking wheels, all other wheels may be consideredfollower wheels.

In a currently preferred embodiment, the logic control circuit 32 alwayssets the paired function 34 as active in response to the initial skidindication. This ensures that the paired function always begins theanti-skid brake process. The vehicle therefore always benefits from thebetter directional control of the paired anti-skid brake control whilethe vehicle is at it's maximum velocity and most susceptible todirectional deviation.

The individual/paired logic circuit 32 may also consider the magnitudeof an indicated slip to determine whether to set the paired function 34or individual function 36 as active. If the indicated slip velocity isexcessive, the logic circuit may override other considerations and setthe individual function as active. An excessive slip velocity may bedetermined by establishing a slip velocity Y which is determined to bethe maximum slip velocity for use with the paired function. Thus, if theindicated slip velocity exceeds the maximum slip velocity for use of thepaired function (i.e. if V_(s)>Y), then the individual function is setactive.

The slip velocity which is deemed excessive (Y) may be determinedthrough several factors. This velocity may vary depending on the needsof the vehicle, as well as the weather and other conditions. Thisvelocity may also be determined dynamically (such as one and one halftimes the initial slip condition or a certain percentage of the vehiclescurrent speed.) Varying this slip velocity may have substantial impacton the performance of the present system.

When the paired function 34 is set, subsequent signals from the leadwheel are sent to both (or all) wheels. Subsequent signals from thefollower wheels are also monitored to determine whether the pairedfunction 34 should be set inactive and the individual function 36 shouldbe set active. Otherwise the signals of the follower wheels are ignoredas long as the paired function is set active.

Two possible conditions exist which would set the paired function 34inactive and set the individual function 36 active in response to theslip signals from the follower wheel. If the slip velocity of thefollower wheel exceeds a maximum slip velocity for use of the pairedfunction (if V_(s)>Y), then the paired/individual logic circuit 32 mayset the paired function inactive and set the individual function active.Similarly, if the slip velocity of the follower wheel exceeds apredetermined slip error, the paired/individual logic circuit 32 may setthe paired function inactive and set the individual function active.

Furthermore, the paired/individual logic circuit 32 may initiate theindividual function 36 in response to any condition which requires thegreatest brake control efficiency. Once a follower wheel exceeds apredetermined slip error or indicates an excessive slip condition theneed for maximum braking efficiency exceeds the need for eliminatingdirectional deviation induced by the anti-lock braking system. In fact,the directional deviation induced by excessive slip conditions mayexceed those induced by the individual function of anti-lock brakingsystem. Under such conditions, the anti-skid braking system of thepresent invention will switch from the paired function to the individualfunction.

With reference to FIG. 3 a schematic diagram of a typical generalaviation aircraft 42 is useful in describing the advantages of thepresent advantages. A general aviation aircraft is one type of vehiclethat may benefit from the use of the present invention.

The wheel base 46 of a general aviation aircraft 42 is the distancebetween main landing gear wheel 1 and wheel 2. These wheels provide themain braking force B₁ and B₂ for the vehicle. Additional braking isprovided for by aerodynamic forces and is represented as vector A.

The center of gravity 44 of a general aviation aircraft is typicallyalong the center-line of the vehicle and somewhat forward of the mainlanding gear. The force of main landing gear braking (as well as otherforces) on the vehicle is conceptualized as if the vehicle isconcentrated at the center of gravity. As long as vectors B₁ and B₂ arethe same, they each create an equivalent moment (calculated as themagnitude of the vector multiplied by the distance from the center ofgravity 44, but in opposite directions. These equivalent but oppositemoments effectively cancel each other. If the braking forces B₁ and B₂are different (such as when differing braking pressure is applied),however, the resultant moments are no longer equivalent. Therefore,differences between the braking forces B₁ and B₂ result in a net momentor torque on the aircraft, which causes directional deviation. Thegreater the wheel base and the greater the difference between thebraking forces, the greater the directional deviation.

This demonstrates the advantages of a paired anti-skid braking system.When the paired function 34 of the present invention is active, thebraking forces B₁ and B₂ are controlled to be equivalent. Therefore,when the paired function is active, there is no net torque on thevehicle and no braking induced directional deviation. This allowsconsistent control of the aircraft particularly at the initiation ofbraking during landing.

Of course, braking force is reduced in slip conditions. When a wheelexperiences slip (V_(w)<V_(ref)) the coefficient of friction between thewheel and surface is reduced. The coefficient of friction is greatestwhile the wheel rolls smoothly across the surface. For this reason, theanti-skid brake control system regulates braking pressure to maintainthe wheels in a rolling condition. When the wheels experience distinctlyvariant slip conditions, optimal braking is accomplished by controllingthe individual brake pressure according to the conditions experienced byeach wheel. Therefore, the present invention is also capable of changingto an individual wheel braking system when needed.

A novel method for controlling anti-skid braking is also disclosed. Thismethod provides for a means to maximize braking efficiency and minimizebraking induced directional deviation within a single control system.This method involves switching the control system between a pairedcontrol system and an individual wheel control system.

Continuous monitoring of the wheels and brakes are part of this novelmethod. The wheels and brakes of a multi-wheeled vehicle, such as ageneral aviation aircraft, are provided with a slip indicator includinga velocity transducer. The velocity transducer continuously monitorswheel velocity. A comparing function may be provided which compares eachwheel velocity signal to a reference velocity for the vehicle. Thiscomparing function provides a signal indicating slip velocity to theanti-skid braking control system.

The first wheel indicating a slip condition initiates the function ofthe paired/individual logic control circuit. This circuit assigns thefirst wheel indicating a slip conditional as the “lead wheel.” In apresently preferred embodiment of a method to control anti-skid braking,the signals from the lead wheel are initially sent to a wheel controlcircuit for controlling each wheel in a paired control function.

The wheel control circuit interprets the slip indication signal andother signals to calculate the optimum braking pressure. These othersignals may include input from the cockpit braking pedals, the referencevelocity of the aircraft and the current brake pressure. An optimumbraking pressure signal is then sent to the brake pressure valves ofeach wheel which are instructed to respond with equivalent brakepressure to each wheel.

While the paired control function is active, the system continues tomonitor each wheel of the vehicle. The signals from the lead wheelcontinue to be sent to the wheel control circuit for optimizing thecontrol of all wheels. The signals from the follower wheel(s) aremonitored for any indication of deep slip conditions. An indication of adeep slip condition triggers the paired/individual logic circuit to setthe paired function inactive and set the individual function active.

Whether a slip condition indication by a follower wheel is deep enoughto change control functions may be determined in at least two ways. If afollower wheel indicates a slip condition exceeding a reference slipvelocity, this may trigger the change. Similarly, if a follower wheelindicates a slip condition exceeding the slip condition of the leadwheel, this may trigger the change. Either or both of these conditionsmay be utilized in the present method. Finally, any condition thatindicates unacceptably inefficient braking may trigger the change.

While the individual function is active, the paired/individual logiccircuit sends the slip indication signals from each wheel to a separatewheel control circuit. Each of these circuits then calculates theoptimum braking pressure for each wheel. These signals are sent to eachwheel separately for optimum braking efficiency of that wheel.

While the individual function is active, the system continues to monitorthe signals sent by each wheel. These signals are employed tocontinually update the individual braking control of each wheel.

In a presently preferred method, once the individual function is set itcontinues to control the anti-skid braking until the vehicle comes to arest or the anti-skid braking is otherwise discontinued. Other methodsmay be utilized to change the system back to a paired function based oncertain conditions. Similarly, methods may be employed which initiatethe anti-skid braking using the individual function. Thus, the presentlydescribed methods may be modified to meet the needs of the differentvehicles or different conditions.

It should be apparent from the foregoing that the presently describedsystems and methods is applicable to various types of vehicles.Aircraft, automobiles, trucks and trains all have the need for some typeof anti-skid braking control. The present invention can readily be usedon any such vehicles.

It will also be apparent from the foregoing that while particular formsof the invention have been illustrated and described, variousmodifications can be made without departing from the spirit and scope ofthe invention. Accordingly, it is not intended that the invention belimited except as by the appended claims:

What is claimed is:
 1. In a method of reducing skid of a wheeled vehiclehaving a plurality of wheels, the method including the steps ofcomparing wheel velocity of each of said plurality of wheels with areference velocity of the wheeled vehicle, and generating a slipcondition signal for each of said plurality of wheels indicating a slipcondition if the wheel velocity is lower than the reference velocity ofthe vehicle, the improvement comprising the steps of: designating one ofsaid plurality of wheels for which said slip condition signal firstindicates an initial slip condition as a lead wheel, and designatingeach of the remainder of said plurality of wheels as a follower wheel;determining an optimum anti-skid braking response for said lead wheel;controlling skid on each of said plurality of wheels in unison inresponse to said initial slip condition based upon said optimumanti-skid braking response for said lead wheel; determining an optimumanti-skid braking response for each said follower wheel; controllingskid on each of said plurality of wheels individually if the wheelvelocity of one of said remainder of said plurality of wheels is lowerthan the reference velocity of the vehicle based upon said optimumanti-skid braking response for each said follower wheel.
 2. The methodof claim 1, further comprising the step of continuously monitoring slipconditions of each of said plurality of wheels.